1. Field of the Invention
The present invention relates generally to television receivers and more particularly to a television receiver suitable for use in a video signal display apparatus having a large display area such as a video projector and the like.
2. Description of the Prior Art
Recently, in a video signal display apparatus having a large display area such as a video projector and the like, its resolution has been increased significantly by improvements of a television receiver (cathode ray rube and electric circuit) and lenses which construct such display apparatus. However, as the resolution is increased, the scanning lines which, up to that time, were inconspicuous become visible or conspicuous, thus making it difficult to improve picture quality.
In the visual display according to the interlaced system, in the NTSC system employing 525 scanning lines per frame, 262.5 lines constitute one field, that is, there are an odd and an even field in each frame, and the fields occur at the frequency of 60 Hz to suppress a field flicker. On the other hand, in order to obtain a desired vertical resolution, as shown in FIG. 1, during each field next to, or immediately following or preceding a certain field, an electron beam scans a scanning line which is spaced 1/2 line interval apart from the corresponding scanning line in the preceding field to provide an interlaced display. In FIG. 1, references lo and le respectively designate scanning lines of odd and even fields, while references Bmo and Bme respectively designate positions of electron beams for odd and even fields in the case of an interlaced display.
In this case, although macroscopically the number of picture images is 60 pictures/second, microscopically light is emitted from each scanning line at every 1/30 second so that its display period is 1/30 second. For example, noting a point P in FIG. 1, the brightness of the point P becomes high at every 1/30 second as shown in FIG. 2. As a result, the visual display of each scanning line is perceived as a flicker in the visual sense.
As a method effective for removing the flicker, it has been proposed to form the electron beam Bmo (Bme) so that its landing spot is elongated such that it lies across both a scanning line lo of, for example, an odd field and an adjacent scanning line le of the even field. In FIG. 3, when considering the point P in the same way as in FIG. 1, the brightness of the point P becomes high at every 1/60 second as shown in FIG. 4. Consequently, if the electron beam Bmo (Bme) is formed so as to have an elongated landing spot, the respective scanning lines lo and le emanate light at every 1/60 second and hence the flicker is much less perceived.
It is, however, impossible to elongate the electron beam Bmo (Bme) landing spot in the vertical direction and to scan the whole of the fluorescent screen. Particularly in the four corners of the fluorescent screen, as shown in FIG. 5, the electron beam Bmo, for example, is distorted and its landing spot becomes longer slantwise so that the signals are overlapped in the horizontal direction to thereby cause the horizontal resolution to be deteriorated. Moreover, when the electron beam Bmo (Bme) is formed longer longitudinally, as shown by a broken line in FIG. 6, the inclination TR of the brightness cross-section of the electron beam loses steepness to affect not only an N scanning line but also an (N+1) scanning line of, for example, the same field and thus the vertical resolution is deteriorated. A solid line in FIG. 6 shows a brightness cross-section of an electron beam with an ordinary shape.